High power density in-wheel motor could be achieved by integrating a high-speed ratio (larger than 25) compound power-split mechanism (CPSM) with small motors. However, due to the exhaustive searching method adopted by the traditional lever analogy method, it is time-consuming to design high-speed ratio compound power-split mechanism configurations satisfying the high power density of in-wheel motor. In this paper, an improved lever analogy method is proposed to find the optimal configurations with a high-speed ratio to satisfy the high power density in-wheel motor. In this method, a judgment algorithm about the rank of structure matrix is proposed to identify three-node compound lever models of the CPSM. The improved lever analogy method can filter out useless configurations that significantly improve the calculation efficiency. The simulation results show that the calculation efficiency is improved by 215 times higher than that of the lever analogy method. Finally, 16 reasonable and 14 new configurations are obtained. This indicates that the improved lever analogy method can provide an effective way to design the high-speed ratio CPSM, which is widely used in-wheel motor-driven vehicles.